The present invention relates to a projection television system.
In a projection television system three cathode ray tubes having phosphor screens luminescing respectively in red, green and blue are arranged either side by side in a planar array or on three sides of a square with dichroic mirrors in the centre to direct light from the two facing tubes in the same direction as the light from the centrally disposed third tube.
Existing projection television systems are regarded as having (1) a moderate to poor resolution, (2) a moderate luminosity and (3) a moderate colour rendition. The reasons behind this are that the tube formats are so small, typically between 5 and 7 inches, necessitating: (1) a very high electron-optical resolution on the phosphor screen area, (2) a very high electron beam loading to generate sufficient light after magnification on the large projection screen, and (3) phosphors with a sufficient linearity at these high electron beam loading conditions. At these high phosphor loads the light output does not proportionally increase with beam current due to saturation. This effect is most pronounced for the widely used ZnS:Ag blue phosphor but to a lesser extent it also occurs for the green Tb activated phosphors, such as YAG:Tb, Y.sub.2 SiO.sub.5 :Tb, LaOC1:Tb and InBO.sub.3 :Tb, and for the red Y.sub.2 O.sub.3 :Eu phosphor commonly used for projection TV. At a characteristic beam current in the green tube of 1 mA (above the typical average instantaneous current of 0.5 mA and below the maximum current of 2 to 2.5 mA), the saturation in the blue tube is so severe (typically a factor of 3 to 6 decrease in efficiency as compared to the linear efficiency; 3 for poor resolution, 6 for good resolution i.e. small spot size) that a significant amount of defocusing of the electron beam spot is necessary in that tube in order to get white-D luminance on the projection screen.
The required high beam currents also lead to increased ageing of the phosphor.
Additional problems are:
(1) the too high x-colour co-ordinate on a chromaticity chart relative to the European Broadcasting Union's recommendation of all green Tb activated phosphors and the too low y-colour co-ordinate of most of these phosphors (InBO.sub.3 :Tb is an exception, it has the correct y coordinate) yielding a moderate colour rendition on the projection screen,
(2) the broadband spectral emission of the blue ZnS:Ag phosphor and the several spectral line emission of the green Tb activated phosphor. Upon imaging on the projection screen this broadband or multiline emission gives rise to a considerable loss of resolution due to chromatic aberration of the projection lenses.
In order to mitigate the luminance, colour rendition and chromatic aberration problems it has been proposed for example in U.S. Pat. No. 4,683,398 to provide interference filters between the phosphor layer and the glass faceplate. The interference filters improve the light output of the display tubes particularly for the red and green phosphors which have line spectra with one or a few predominant lines. The gain in light output is typically a factor of 1.5 to 1.9 for these phosphors. In addition the colour rendition in the green improves and the chromatic aberration decreases. The improvement in light output is less (about 1.3) in the case of the blue phosphor ZnS:Ag because of its broadband spectrum. With respect to light output blue is already the bottleneck when no interference filters are used. Because the gain factors due to the application of interference filters are larger in the green and red, blue essentially limits the achievable luminance both without and with interference filters.
In order to increase the light output from the blue light emitting phosphor in a conventional projection television system, i.e. without interference filters, it has been proposed to defocus vertically the spot produced at the screen of the blue tube so that each line is effectively scanned two to four times in succession when raster scanning. This leads to a lower instantaneous phosphor load and because the blue ZnS:Ag phosphor decays in about 10 us, much shorter than a typical line repetition time of 64 us, it leads to much less saturation and to a higher light output per mA of beam current. However this is accompanied by a loss in resolution in the vertical direction corresponding to the amount of defocusing. Resolution, particularly in projection television tubes, must be very high in order to resolve satisfactorily the image at the projection screen.
Multibeam (or multispot) display tubes for use in a variety of applications are known, and are described by way of example in a paper "Multiple Beam Cathode Ray Tube Design Overview", presented by Vernon D. Beck and Bruce P. Piggin at the 1985 International Display Research Conference, in U.S. Pat. Nos. 3,714,489 and 4,259,692, and in U.S. Pat. application Ser. No. 110,054.
U.S. Pat. No. 4,259,692 discloses providing a multispot projection television tube to reduce cathode output limitations, space charge and phosphor non-linearity problems. Three vertically separated spots are produced and are modulated with the aid of delay lines so that the same information is given to each spot when at a corresponding position in the same line. However, there is no suggestion of having a multispot display tube with other single spot display tubes.